A balanced flue is *room sealed*, that is the whole point.

Balanced flue has inner & outer tube, IIRC...
- Inner allows air to enter for combustion ONLY
- Outer allows heated combustion products out ONLY

The only draught would be room convective draught caused by air
flowing over the heat exchanger.


Class-1 chimney gas fire is conversely very different.
- Some are only 45-55% efficient (some go 72%+)
- Thus 28-55% of their heat goes up the chimney
- Air is drawn through through the house via house vents
- Thus they can cause a howling draught under a door

The most common cause of a howling draught is a full bore chimney pot
without rain guard, being sucked by the prevailing wind, combined with
the huge thermal mass of the chimney length. That draught can spin a
newspaper from front to back without difficulty if placed near a
doorway and reduces efficiency yet further - it is how a gas fire can
"suck a house cold".

The often forgotten trick with open hearth fires or gas fires when out
is that they STILL suck air all night long, that thermal mass &
prevailing wind can combine to "suck the house cold". Stick a sausage
outside the relevent door when going to bed, you may find the house is
significantly warmer.

This is why some people end up with Class-1 flame-effect or gas-fires
AND central heating which "fight one another", the central heating is
having to preheat the air so as to avoid draughts which vanish up the
chimney. A door sausage can save £50/yr and avoid such problems - BUT
there must still be sufficient air for combustion (just some chimney
draw like an 9" square extractor on maximum without a fire).

So any "draught" was due to convective effects across the balanced
flue fire's heat exchanger, or it was a class-1 flue type fire (using
chimney).